1. Field of the Invention
The present invention relates to novel thermoplastic, elastomeric block copolymers which comprise a plurality of recurring units, each of which recurring units comprising a rigid polystyrene block and a flexible diorganopolysiloxane block, such blocks being joined together via a non-hydrolyzable chemical bond.
The invention also relates to a process for the preparation of such block copolymers by hydrosilylation.
2. Description of the Prior Art
Thermoplastic elastomers comprising polystyrene blocks and diorganopolysiloxane blocks are known to the art. Compare U.S. Pat. Nos. 3,678,125 and 3,678,126. Same are prepared via a multi-stage process, for example:
(1) The anionic polymerization of styrene, or suitable derivatives thereof, such as .alpha.-methylstyrene, in a solvent medium with the aid of an organic dilithium compound;
(2) Contacting, at a temperature below -50.degree. C., the polystyrene polymer prepared in stage (1), bearing a C-Li group at each end of its chain, with a minor amount of a hexaorganocyclotrisiloxane;
(3) Adding the hexaorganocylotrisiloxane (in an amount sufficient to obtain the desired molecular weight of the diorganopolysiloxane block) to the polymer obtained in stage (2), bearing a Si-O-Li group at each of its chain, and then heating same in order to effect anionic polymerization of the hexaorganocyclotrisiloxane;
(4) Next coupling the polystrene-polydiorganosiloxane copolymer prepared in stage (3) with a diorganodichlorosilane; and
(5) Washing, with water, the copolymer obtained in stage (4), in order to replace the terminal Si-O-Li groups by Si-OH groups, and, simultaneously, to eliminate lithium chloride, or, alternatively, by reacting a silane of the formula T.sub.3 SiCi with such Si-O-Li moieties to form terminal groups Si-O-SiT.sub.3 (T=hydrocarbon radical).
The aforesaid process, however, is not easy to carry out on an industrial scale. In fact, it requires considerable time, strict supervision throughout the various stages and the use, in stage (1), of a very expensive organic dilithium compound. This dilithium compound is essential because it effects formation, in stage (2), of the terminal groups Si-O-Li which will ensure, in stage (3), the anionic polymerization of the hexaorganocyclotrisiloxane without causing rearrangements or scissions of the polysiloxane chains. Therefore, the terminal groups Si-O-Na or Si-O-K, which could be obtained from dipotassium or disodium organic compounds which are more readily available and are considerably less expensive, cannot be used in this multi-stage process.
A further disadvantage of the noted prior art process also arises from the lack of reactivity of the hexaorganocyclotrisiloxane with respect to the groups ##STR4## Li blocking the chain of the .alpha.-methylstyrene polymers.
Since these living .alpha.-methylstyrene polymers are not stable above -40.degree. C., and since they only react with the hexaorganocyclotrisiloxane above this temperature to form the terminal groups Si-O-Li, it is necessary to add styrene thereto before they are brought into contact with the hexaorganocyclotrisiloxane, so as to include small polystyrene blocks at each end of their chain. This technique makes it possible, ultimately, to operate at a temperature above -40.degree. C. without depolymerization taking place, but it has the disadvantage of introducing heterogeneities into the chain of the .alpha.-methylstyrene polymers.
Accordingly, serious need exists in the art to enable manufacture of other kinds of thermoplastic elastomers comprising polystyrene blocks and diorganopolysiloxane blocks by a simpler and yet more reliable process, desirably by using not only organic dilithium compounds, but also organic disodium and dipotassium compounds.
Cf. Farbe Und Lack, 70, No. 4, 249-258 (1964) wherein the preparation of a low molecular weight copolymer is described, such copolymer having poor mechanical properties and being prepared by adding an allyldimethylsiloxane terminated polystyrene to a dihydrogenopolydimethylsiloxane.